CPU baking only uses a fraction of the CPU, starting with 2.90
I found the reason! We were setting the tile size to the size of the map. According to the comment next to the code it was an optimisation recommended on Blender forums, and indeed it was way faster than setting a small tile size on 2.83. On 2.90 however, it was sluggish as hell. Setting a 32 tile size made things much better (it's even a wee bit faster than with 2.83)